Variety: the principle of design concerned with difference or contrast.
Our lives would be very boring without variety. Imagine how our world would be if everything were exactly the same all the time. Imagine having the same cereal for breakfast every day, wearing the same clothes every day, listening to the same song every day, etc. It wouldn't be long before we'd be screaming for something different. Variety helps us keep things interesting in our lives, and it does the same for art.
In art, variety is used to create interest and draw attention to something. As a comparison, think of the last TV commercial you saw for a specific product. How long has that particular commercial been on the air? Have there been other ads for the same product? How many? The advertiser uses new ads for their products to maintain interest in their products. If they relied on the same ad for their product, people would lose interest quickly.
*Legos almost went out of business until they added in the right type of variety!